1. Field of the Invention
This invention relates to a microscope objective, and in particular, to a high-NA, high-magnification, and infinity-correction type objective used in a deep ultraviolet region corresponding to a wavelength of approximately 250 nm.
2. Description of Related Art
It is known that objectives used in a deep ultraviolet (DUV) region corresponding to a wavelength of approximately 250 nm are roughly divided into three types. The first type objective is constructed with only a plurality of lenses made with the same medium (quartz, mostly), and is designed so that chromatic aberration cannot be corrected in theory (refer to, for example, Japanese Patent Kokai Nos. Hei 6-242381 and Hei 10-104510). The second type objective is constructed so that lenses made with different media (quartz and fluorite, mostly) are cemented together with an adhesive and chromatic aberration can be corrected (refer to, for example, Japanese Patent Kokai Nos. Hei 5-72482, Hei 9-243923, Hei 11-249025, and 2001-42224). The third type objective is designed so that, of a plurality of lenses, a lens made of quartz and a lens of fluorite are used to correct chromatic aberration, but so that both lenses are not cemented with the adhesive (refer to, for example, Japanese Patent Kokai Nos. Hei 11-167067 and 2001-318317).
The resolution of a microscope is fundamentally determined by a wavelength and the numerical aperture of the objective. The median wavelength of visible light used in an ordinary microscope is approximately 550 nm and the maximum numerical aperture of a dry objective is about 0.9. Therefore, when a wavelength to be used is set to around 250 nm, the resolution is roughly doubled because the wavelength is halved. However, this is limited to the case where the numerical aperture remains unchanged. With a wavelength of about 0.4, even though the wavelength to be used is set to around 250 nm, both the wavelength and the numerical aperture are halved, and thus the resolution is counteracted and is exactly the same as in a conventional microscope.